I can understand having a reasonable period of time to conclude contract negotionations, movie deals, and the like. And I can understand why publishers would want a reasonable period of time to make their money back.
What I can't see is allowing publishers a lifetime plus 70 years to make their share.
20 - 25 years, allowing extension for 5 year period, file fee for the extension should double every time its used. So if the first extension is $5k, the second would be 10k, the third 20k, etc. Quickly reaching the point where it is not worthwhile for most books.